[tune+rllib] Enforce input type for Experiment(config) to handle rllib AlgorithmConfigs

[justinvyu]

Why are these changes needed?

#42116 resulted in params.json saving the wrong data due to dumping an AlgorithmConfig object rather than a dict, which is the expected type for Experiment.config.

The fix should be to force the user to call AlgorithmConfig.to_dict to satisfy the Ray Tune API, rather than adding more rllib specific instance checks in Ray Tune, as is done in this other PR. The _Config class is a hack that is used in one place, but we should not add more of this class throughout the codebase when possible.

The fix to the user post linked below:

experiment_spec = tune.Experiment(..., config=ppo_config.to_dict())`
tune.run_experiments(...)

Related issue number

#41370

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[matthewdeng]

Makes sense; chances are there other places in the codebase that follow the Dict typehint.

[justinvyu]

@sven1977 @simonsays1980 @ahmedammar Just a heads up, I switched to an alternate solution that would involve fewer changes in Ray Tune. See the PR description for the new recommended usage (converting to dict before passing into Ray Tune APIs). Ray Tune expects a dict in many other places, and we should not do this typecheck in every case.